Block copolymer lithography has emerged as an alternative lithographic method to achieve large-area, high-density patterns at resolutions near or beyond the limit of conventional lithographic techniques. Block copolymers are polymeric chains formed by two or more immiscible polymer blocks within the same chain. Free energy minimization induces micro-phase separation of the immiscible, but bonded, blocks, resulting in self-assembled periodic and regular patterns. Thin films of block copolymers can form 2-dimensional arrays of regular, periodic patterns on a substrate. Upon selective removal of one of the block materials from an assembled block copolymer thin film, the remaining portion of the film can act as a lithographic mask. The most commonly used block copolymer structures for lithographic applications are thin films of spherical, cylindrical or lamellar domains with cylindrical and lamellar patterns perpendicularly oriented with respect to the surface of the substrate.
Selective removal of one of the constituent blocks is usually accomplished by a wet developing process where one block is selectively degraded and then the sample is immersed in a selective solvent that dissolves only the degraded block, leaving the other block unchanged. Upon drying, the undissolved block stays on the substrate forming a suitable mask for lithography. However, pattern collapse is a common problem during the drying step for critical dimensions under about 30 nm. A possible solution to pattern collapse in lamellar phase block copolymer has previously been achieved by performing a dry developing step in which oxygen plasma is used to etch away one of the two blocks. However, the etch selectivity is poor and as the material to be removed is etched away, the block that is intended to remain also etches down resulting in a thin, rounded mask that is not adequate for high fidelity lithography. A wet developing process that preserves the entire structure of the undissolved material is preferred if pattern collapse can be avoided.
Achieving 3-dimensional structures suitable for lithography (e.g., with selective removal of one material from the 3-dimensional stack) has also remained a challenge for self-assembly.